The present invention relates to the art of earth boring and, more particularly, to a raise boring head for boring raise holes in a mine by enlarging a pilot hole into a raise hole having a larger diameter than the pilot hole.
A relatively large diameter hole may be provided between a first location and a second location in a mine or other underground works by an operation commonly referred to as raise drilling. A raise drilling operation begins by drilling a small diameter pilot hole through the earth from a first location to an opening at the second location using a small diameter pilot bit. After the pilot hole is completed, the pilot bit is removed from the drill string and a large diameter raise bit or raise head attached. The raise head is rotated and drawn along the pilot hole, thereby enlarging the pilot hole to the desired size.
As shown in U.S. Pat. No. 3,659,659 to Carl L. Lichte, patented by May 2, 1972, raise bits of the prior art generally include a bit body positioned about a central bit axis with rolling cutters mounted at various distances from the central bit axis for disintegrating the earth formations. The rolling cutters may be locked in place on the bit by various locking mechanisms. For example, locking mechanisms are shown in U.S. Pat. No. 3,203,492 to C. L. Lichte, patented Aug. 31, 1965; in U.S. Pat. No. 3,705,635 to William M. Conn, patented Dec. 12, 1972; and in U.S. Pat. No. 3,612,196 to Robert L. Dixon, patented Oct. 12, 1971. The cutters may be positioned to cut the working face according to various geometries. For example, cutter locations are shown in U.S. Pat. No. RE 27,597 to M. L. Talbert, patented Mar. 13, 1973, in U.S. Pat. No. 3,805,901 to William D. Coski, patented Apr. 23, 1974, and in U.S. Pat. No. 3,638,740 to Dan B. Justman, patented Feb. 1, 1972. A lubrication system may be provided to transmit lubricant to the bearings of the rolling cutters, as shown in U.S. Pat. No. 3,675,729 to William J. Neilson, patented July 11, 1972.
At the completion of many raise drilling operations it is necessary to lower the large diameter raise head back through the entire length of the hole that has been bored. For example, the earth boring machine that is drawing the raise head upward may block the area needed for completing the large diameter hole. In such cases the raise head is lowered back through the previously bored hole and the raise head removed. The earth boring machine is then removed and the earth formations broken through to complete the large diameter hole. The lowering of the raise head through the previously bored hole can be a very difficult procedure. The raise head may become stuck in the hole by being lodged on portions of the formations through which the hole extends. A raise boring head capable of being reduced in diameter prior to being lowered back through the previously bored hole would substantially reduce the risk of the raise head becoming stuck in the hole.
In addition, it is often necessary to lower the raise head down a partially completed raise. This may be necessary in order to change damaged or worn-out cutters or to remove and replace the raise head. Lowering of the raise head can be difficult and time consuming because the gage cutters have a tendency to hang up on the wall of the hole as the head is lowered. This is particularly true in non-vertical holes. Lowering the head can cause considerable damage to the gage cutters because the surf area of the cutters scrub the wall on the way down. In addition, the unsupported drill pipe and stabilizers are subjected to unusual loads which are believed to be prime factors in some premature drill string failures. After new cutters are installed and the raise head starts back up the raise hole, the new cutters will generally cut a slightly larger hole than the old, worn cutters did. The new gage cutters must ream this small amount of hole which causes excessive wear on the outer rows of the new gage cutters.